To enhance adsorption capacity of wheat straw (WS) toward copper ion from solution, carbon disulfide was used to modify WS by a facile grafting method through epichlorohydrin and ethylenediamine. So WS containing xanthate groups (XWS) was obtained. The XWS was characterized using elemental analysis, X-ray diffraction, infrared spectroscopy and adsorption property of XWS toward copper ions. The results showed that S was introduced into the surface of WS. The solution pH was in favor of Cu2+ adsorption at pH 5, while NaCl existing in solution was slightly favorable for adsorption. The adsorption kinetic followed the pseudo-second-order kinetic model, while the adsorption isotherm curve was well fitted using the Langmuir model. The adsorption capacity was 57.5 mg·g−1 from experiment. The process was entropy-produced, endothermic and spontaneous in nature. The column adsorption was performed and Yan model was good to predict the breakthrough curve. XWS as adsorbent is promising to remove copper ions from solution, and this offers one way of effective utilization of waste byproduct from agriculture.
Xanthate wheat straw was prepared and characterized.
Adsorption capacity was significantly improved after modification.
Adsorption performance was carried out in batch and column modes.
The breakthrough curves in column mode can be fitted by Yan model.
With the rapid growth of the global population and the rapid development of modern industry, the human demand for water resources is increasing at an alarming rate, but at the same time, the pollution of water resources is becoming increasingly serious. Heavy metal pollution has become one of the most harmful water pollution problems in recent years (Fu & Wang 2011). Generally, heavy metals in environmental pollution refer to biological or toxic heavy metals such as mercury, cadmium, lead, chromium, zinc and copper. The heavy metal ions of pollutants come from the following industries: mining, electroplating, paper making, batteries, fertilizers, leather, pesticides, etc. (Yu et al. 2020). The water has a certain self-purification function. After a small amount of heavy metal ions enter the water body, the water body can be treated by itself without affecting the environment. However, with the development of industry, a large number of heavy metals enter the water body. Heavy metal ions are not biodegradable and can accumulate in organisms. Copper is an essential trace element in human body and an integral part of various enzymes. However, if copper ions exceed a certain threshold in the body, they will cause liver, kidney, capillary and central nervous system damage.
The removal methods of heavy metal ions in water mainly include precipitation, ion exchange, membrane filtration, photocatalytic degradation, oxidation, flocculation and adsorption (Fu & Wang 2011; Hashim et al. 2011). The adsorption method has many advantages, such as being suitable for the treatment of dilute solution, high enrichment multiple, good separation effect, high product quality, easy liquid-solid separation, no secondary pollution and emulsification, and adsorbents can be recycled, so it has become one of the most widely applied and promising technologies (Zhou et al. 2019; Yu et al. 2020).
Compared with activated carbons, zeolites, fibers, organic resins and so on, adsorbents from a variety of agro-forestry wastes have been applied in wastewater treatment because of their advantages: easy to obtain, low cost, easy operations, and availability (Zhang et al. 2014).
At present, straw and many plant materials, such as banana leaves, potato skins, cocoa shells, rye straw, corn cobs, peach shells, discarded tea leaves, etc. are selected as adsorbents to remove pollutants from water (Bhatnagar & Sillanpaa 2010). Wheat straw (WS) is one by-product of agricultural production; there is the natural synthesis of large amounts every year. WS is often disposed of as waste, and this does not lead to effective use of this resource. So it is necessary to develop one way for full utilization of these materials. The main components of WS are cellulose, hemicellulose, lignin, etc., and it can be directly applied as adsorbent. Raw WS was used to treat aqueous solutions containing Cu2+, Zn2+ and Ni2+ and the maximum adsorption capacities were 5 mg·g−1, 2.5 mg·g−1 and 3.25 mg·g−1, respectively (Gorgievski et al. 2013). Although straw has a certain adsorption capacity for pollutants, its adsorption capacity is relatively weak. In order to improve the adsorption property of WS, a simple chemical modification is needed. Amine group-modified agro-wastes have been studied a lot. Amino groups are used to enhance the adsorption properties of anionic dyes and metal ions (Xu et al. 2011; Shang et al. 2016; Dong et al. 2019; Li et al. 2020). Zhong et al. prepared amphoteric wheat stalks by reacting epoxy wheat stalks with ethylenediamine, triethylamine and chloroacetic acid, studied the adsorption of Cu(II) and Cr(VI) on modified wheat stalks, and fitted them with kinetic model and adsorption isothermal model (Zhong et al. 2014). Ethylenediamine and triethylamine, diethylenetriamine, dimethylamine, etc. are often considered to modify materials (Deng & Ting 2005). Disulfide carbon can react with amino groups in basic conditions and xanthate can be obtained. Xanthate can effectively bind some heavy metal ions, such as Cu2+, Pb2+ and Hg2+. So disulfide carbon modified materials can be prepared and the adsorbents with xanthate group can selectively remove metal ions (Chauhan & Sankararamkrishnan 2008; Liang et al. 2009). But there is no study about WS as adsorbent in this direction.
In this paper, xanthated wheat straw (XWS) was prepared with reaction between CS2 and ethylenediamine grafted to surface of WS through epichlorohydrin. The adsorption property of Cu2+ by XWS was presented in batch mode, while breakthrough curve was also discussed. Copper ion is selected in this study as copper is widely used in various industries and has negative effects on environment and human beings when there is high level of copper ions in the effluent. Regeneration performance of the spent adsorbent was also explored.
MATERIALS AND METHODS
Instruments and reagents
The WS was obtained from Zhengzhou suburb. All of the chemical reagents used in this research, including epichlorohydrin (ECH), ethylenediamine (EDA), CS2, Na2SO4, CaCl2, NaCl, HCl, Cu (NO3)2·3H2O, NaOH and 95% ethanol, all of analytical purity, were supplied by Sinopharm Chemical Reagent Co., Ltd, China. The laboratory water was deionized water.
Stock solution of Cu2+ was prepared by dissolving Cu(NO3)2·3H2O in deionized water, and the further working solution was obtained by daily dilution to a suitable concentration. Solution pH was adjusted using different concentrations of HCl and NaOH.
Instruments used in this research are the following: pH-mV meter (LeiCi PHS-3S, China), air bath shaker (GuoHua SHZ-82, China), UV spectrophotometer (Shimadzu brand UV-3000), X-ray diffraction (D/MAX-RA, Rigaku, Japan), Fourier infrared spectrometer (PE-1710FTIR, American PE Company), X-ray fluorometer (S4PIONEER, Germany, Brock Company), scanning electron microscope (JSM-6700F, Nippon Electronics), elemental analyzer (Flash EA 1112, Thermo Electron Corporation), peristaltic pump (BT100-2 J, Baoding Langge Constant Current Pump Co. Ltd), mill (JL-02A, Shanghai Jiarui Tools Co. Ltd).
Preparation of absorbent
The preparation of ethylenediamine modified WS (EDA-WS) was through grafting reaction using ECH (ECH modified WS, ECH-WS) followed previous work (Wang et al. 2011). Next step was to modify the EDA-WS with CS2 via xanthan acidification reaction. Put the EDA-WS into a 500 mL conical flask, add 200 mL 1.0 mol·L−1 NaOH, 12 ml CS2 and 40 mL 95% ethanol, and place the conical flask in a thermostatic oscillator to oscillate for 5 h at 313 K. The xanthan acidified straw (XWS) is filtered and washed with ethanol to remove the unreacted CS2. The modified straw is again washed with distilled water to neutral at 353 K before being dried at 60 °C in an oven.
The chemical reactions in the modification process are as follows:
Characterization of materials
Several analytical techniques were used to characterize the adsorbents. The pH at point zero charge of WS and XWS was obtained by the 0.01 mol·L−1 NaCl solid addition method. Fourier transform infrared spectroscopy (FTIR Spectrometer, Nicolet iS50, USA) was used to confirm the characteristic functional groups of WS and XWS. The contents of nitrogen, carbon, hydrogen and sulfur elements were determined by automatic element analyzer (Flash EA 1122, Thermo Fisher Scientific). The crystal textures were imaged by X-ray powder diffractometer (D/MAX-RA, Japan).
Adsorption experiments in batch mode
The removal of Cu2+ from aqueous solution by XWS was studied on a Thermostatic shaker (SHZ-82) at constant speed (120 rpm) in batch mode. A certain amount of absorbent (10 mg) was placed in a 50 mL Erlenmeyer flask, into which was added 10 mL of Cu2+ of initial known concentration (50 mg·L−1 except adsorption isotherm study) with solution pH 5.0 (except effect of solution pH) at 303 K. After adsorption, mixtures were filtered and the residual Cu2+ concentration in filtrate was measured using atomic absorption spectrophotometry at wavelength of 324.7 nm (Perkin–Elmer, AAanalyst 300).
The exhausted or spent adsorbent (Cu2+ loaded XWS) was obtained for the adsorption of 50 mg L−1 Cu2+ at pH 4. Then, Cu2+ loaded XWS was washed with distilled water to removal any unabsorbed Cu2+ and was dried at 353 K. The exhausted adsorbent was regenerated by 0.1 mol·L−1 HCl solution.
Adsorption test in a fixed-bed column was carried out in one glass column (1 cm ID and 25 cm height), packed with 0.90 g XWS (height 5.4 cm). Solution with 40.0 mg L−1 Cu2+ was pumped in down flow mode at flow rate 9.9 mL·min−1 using a peristaltic pump. Samples from column effluent were collected at regular intervals to detect concentration of Cu2+. Then breakthrough curve (Ct/C0 ∼ t) can be obtained.
RESULTS AND DISCUSSION
Characterization of materials
The pH of point zero charge of materials is helpful to confirm the surface charge at various solution pH values. The results of measurement of pH of point zero charge (pHpzc) for WS and XWS are shown in Figure 1. It was observed that pHpzc was 7.7 for WS and 8.8 for XWS. There is positive charge on the surface of material at solution pH lower than pHpzc, while there is negative charge at the solution pH above this value. The improvement of pHpzc after xanthan acidification indicates that the surface properties were changed.
The percentages of common elements are acquired by elemental analysis and the results are listed in Table 1. There is only sulfur (4.81%) for XWS, respectively. Compared with WS, the increased percentages of N and S were from CS2 and EDA modification.
|Sample .||N .||C .||H .||S .||qe (mg·g−1) .|
|Sample .||N .||C .||H .||S .||qe (mg·g−1) .|
The X-ray diffraction (XRD) patterns of WS, ECH-WS, EDA-WS and XWS are presented in Figure. 2, suggesting that WS has two diffraction peaks around 16.5° and 22°, respectively, which are generated by the low crystallinity polysaccharide structure and the high crystallinity of cellulose. The diffraction patterns of ECH-WS and EDA-WS are basically the same. There is a strong diffraction peak near 20.3°, and the diffraction peak at 20.3° is also generated by the high crystallization of cellulose. The diffraction peaks of XWS near 20.63° and 30.9° were also generated by the high crystallization of cellulose (Sánchez et al. 2016). By comparing the intensity of diffraction peaks of these three kinds of WS, it can be concluded that the intensity of diffraction peaks of the modified WS is in all cases smaller than that of WS, which also indicates that the crystal structure of the modified WS is destroyed and the order of crystal structure is reduced.
The FTIR analysis can offer some information about functional groups on the surface of materials (Chu et al. 2020). FTIR spectra of WS and XWS are presented in Figure 3. There is cellulose, lignin, etc. in plant materials. It is obviously seen from Figure 1 that FTIR analysis of WS and XWS displayed a number of absorption peaks, reflecting the complexity of the materials. There were −OH (peak at 3,416 cm−1), carbonyl group (peak at 1,735 cm−1), C − O − C (peak at 1,061 cm−1), −CH2 (peak at 1,061 cm−1) etc. on the WS surface from FTIR analysis (Han et al. 2010). For XWS, the peak at 1,735 cm−1 was not observed, as ethylenediamine can react with carbonyl groups. A new peak for XWS at 2,065 cm−1 was formed by the stretching vibration of −SH, which also indicated that −SH was successfully introduced into the surface of WS; that is, sulfur modified WS was successfully obtained. After modification, the structure of WS is relatively stable.
Adsorption study in batch mode
Effect of contact time on adsorption quantity
Reactive time is often of concern during adsorption study. The effect of contact time is depicted in Figure 4. It was clearly noticed from Figure 4 that values of qt increased significantly at the initial stage, and then increased slowly until reaching the adsorption equilibrium after 250 min. In the initial phase, the surface sites were available for adsorption, then the adsorption of Cu2+ was in a gradual adsorption phase, and finally the Cu2+ uptake reached equilibrium (qe to 44.4 mg·g−1). In the next study, contact time is 300 min in order to reach adsorption equilibrium.
Under the same conditions, qe values for WS, ECH-WS and EDA-WS are also listed in Table 1. It was shown from Table 1 that there was highest adsorption capability about XWS for removal of Cu2+. This is due to xanthate existing in the surface, which has good adsorption capacity toward some cationic metal ions.
Nonlinear regression method was selected to fit the kinetic results, and the obtained results are shown in Figure 4.
It was noticed from Figure 4 that there were higher determined coefficient (R2) and lower values of SSE, and the value of qe from this model was not far from the value of qe from experiment. (SSE = Σ(q–qc)2, where q and qc are the experimental value and calculated value from model, respectively.) Furthermore, the fitted curve was also not far from the experimental curve. So it can be concluded that the pseudo-second-order kinetic model can be used to predict the kinetic process of Cu2+ adsorption onto XWS.
Effect of common salts on adsorption
There are common salts in metal ion wastewater, so it is necessary to study the behavior of coexisting salt in solution on adsorption process. The results are presented in Figure 5.
It was observed from Figure 5 that the values of qe increased slightly with the increase in the NaCl concentration, and then adsorption quantity showed no change even with further large salt concentration. So it is implied that the major mechanism is the coordinate action between copper ion and xanthate and there is no interaction force between Na+ and the adsorbent. This is significant regarding no effect of salt on adsorption quantity, and it was inferred that XWS can be applied to bind Cu2+ in real wastewater. It is concluded that complexation, not ionic exchange or electrostatic attraction, is the main mechanism of Cu2+ binding. Based on the fact that the sewage contains other metal ions, NaCl concentration was maintained at 0.02 mol·L−1 in the following experiments.
Effect of solution pH on adsorption
Solution pH is also an important parameter in adsorption process, as pH can affect the protonation and deprotonation of the surface of the adsorbate and adsorbent. When the pH value of the solution is less than or equal to 6, copper ions exist mainly in the form of Cu2+ in the solution. Figure 6 shows the effect of solution pH on Cu2+ adsorption quantity by XWS. Under the condition of strong acidity, the unit adsorption capacity of XWS is relatively small. When pH value increased from 2.0 to 3.0, adsorption capacity increased significantly (from 8.44 mg·g−1 to 26.9 mg·g−1). At pH = 5.0, value of qe was 47.5 mg·g−1. Although the increase of pH value is conducive to adsorption, metal ions are easy to precipitate at pH over 6. The adsorption of positive metal ions is not favored at lower pH of solution.
Adsorption isotherm curve
Some useful information can be clearly obtained from adsorption isotherm curve. The experimental results are depicted in Figure 7. It was noticed that the values of qe for Cu2+ adsorption sharply increased with the increase of Cu2+ concentration, then increased more slowly and reached near equilibrium (57.5 mg·g−1). This may be attributed to the concentration gradient being the driving force for the adsorption of Cu2+ on the XWS.
The Langmuir model is used to fit the equilibrium data using nonlinear regression analysis, and the results are also shown in Figure 7.
It was obviously seen from Figure 7 that the value of R2 was over 0.98, and the error was relatively small; the value of qm was also close to the experimental result. This implied that Cu2+ adsorption onto XWS is dominated by monolayer adsorption and this model can describe the equilibrium process and predict the adsorption capacity.
Effect of temperature on adsorption quantity and thermodynamic analysis of adsorption
At C0 = 50 mg·L−1, the values of qe were 37.5, 44.4, 46.2 mg·g−1 g at 293, 303 and 313 K, respectively. This showed that the process is endothermic and it is in favor of Cu2+ adsorption at higher temperature.
The thermodynamic data of the adsorption of Cu2+ by XWS were obtained by using the thermodynamic formula, as shown in Table 2. According to Table 2, values of parameters (ΔS < 0, ΔH > 0, ΔG < 0) showed that Cu2+ adsorption onto XWS was a spontaneous and endothermic, entropy reduction process. At the same time, value of ΔH value is around 40 kJ·mol−1; adsorption is mainly a coordination effect. So the process of adsorption is mainly coordination.
|ΔH (kJ·mol−1) .||ΔS (J·mol−1·K−1) .||ΔG (kJ·mol−1)|
|293 K .||303 K .||313 K .|
|ΔH (kJ·mol−1) .||ΔS (J·mol−1·K−1) .||ΔG (kJ·mol−1)|
|293 K .||303 K .||313 K .|
Desorption study is not only in favor of explaining the mechanism of adsorption, but also making the process more economical and maybe recovering valuable compounds (Su et al. 2013; Zhao et al. 2017; Liu et al. 2020). The results are 55% of desorption efficiency using 0.1 mol·L−1 HCl solution. This showed that the reaction between Cu2+ and xanthate onto XWS was strong and not easily desorbed.
Adsorption in column mode
The property of Cu2+ adsorption onto XWS can be further carried out by column mode, as this performance is continuous. The breakthrough curve of Cu2+ adsorbed onto XWS is illustrated in Figure 8. It was markedly seen that the shape of curve was ‘S’ type. The values of t0.05 (breakthrough time) and t0.5 (half breakthrough time) (at time of Ct/C0 = 0.5) were 46 min, respectively.
According to Equation (7), the value of qe(exp) was 27.5 mg·g−1.
So value of L was 5.1 cm through calculation, and this was a relatively short length.
Using nonlinear regressive analysis method, the values of a, b, determined coefficient (R2), SSE and fitted curve are also shown in Figure 8. There was a higher value of R2 and lower value of SSE, while the value of q0 (bC0/m) from Yan model was 20.3 mg·g−1, smaller than qe(exp). Moreover, the fitted curve shown in Figure 8 is very close to the experimental curve. It was implied that Yan model can predict the column process.
XWS for the removal of Cu2+ from solution has been successfully prepared though chemical reaction, exhibiting good adsorption capacity. The coexisting common salts were slightly advantageous for Cu2+ adsorption. The effects of column height, flow rate and initial concentration on the adsorption process were studied in dynamic experiments. By investigating the isotherms and kinetics fitting data, it was indicated that Langmuir model and pseudo-second-order kinetic model were suitable to fit the equilibrium data and kinetic data, respectively. The adsorption mechanism of Cu2+ onto XWS is mainly coordination. Yan model could well describe the column curve. So XWS has potential with good adsorption property for removal of Cu2+ from solution.
This work was financially supported by the Henan province basis and advancing technology research project (142300410224).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.